inadequacies...

I re-piped a residential boiler for outdoor reset at work. The way it works is; the control gets input from a sensor that measures outdoor temperature and then uses that to determine the optimal water temperature to use to heat the house. You set the design temperature for your geographic location and the control will give you 180 deg water when it sees that outdoor temp. It came out of the box at 10 deg F so I left it that way because the design temp for Phila, PA is 10 deg F. So on the coldest day of the year (average 10 deg) the boiler will heat the water to 180 deg F. On a 50 degree day it will reset to maybe 105 degrees. Saves fuel without sacrificing comfort. Sounds simple, right?

Except it ain't so simple.

You see, the boiler can't see water temps below 140. Otherwise the flue gasses start to condense and that condensate is acidic and before you know it, no more boiler. So I made it primary/secondary. The boiler and the primary loop run at one temp and the secondary loops at another. In theory. We have yet to see how that works out in practice. I'm counting on the fact that some knucklehead put a 175,000 btu (net) boiler in a house with only 80,000 btus worth of baseboard in two loops. So the boiler should heat up fast enough to keep it out of the condensing zone even though the baseboard water is relatively cool. Putting the water temp sensor on the common return will help.

Already I noticed a problem though. I re-used one of the zone valve bodies from the original piping to try to save money. (They're like $100 each) It's by-passing and over-heating the #2 heat zone. It's a mostly unused sitting room and bathroom for the staff of the home and they like it hot anyway (like 85 degs hot) so no one has noticed or complained yet. They weren't sure if they could complain (justifiably) when I changed the thermostats to ones that max out at 75.  So if it's 78 they're not likely to complain about that. 
If I don't replace that valve body and it continues to bypass it could be,...well, I'm not really sure how it will affect things. I think it will help protect the boiler because more water will have to heat up to the setpoint before the control shuts the burner off. But since it's a shorter zone it could bring the temp of the common return up sooner. Sooner or later I'll have to replace it though. I think.

I haven't actually got the outdoor reset control all the way wired in yet. It doesn't have control of anything yet but the sensors are installed and it has power. I want to spend some time with it watching the temps and tweeking the setpoint once it's controlling water temp. That's when I will know all the weaknesses in the system. The inadequacies in my plan.

I know this much. I doubled the volume of water delivered to the baseboards for half the money the other guys wanted to replace all the baseboard in the house. Double the water volume and you double the btus delivered. Just replacing all the baseboard would not have done much toward increasing the btus delivered. Not unless you separated the two zones. The way it was piped before all the water had to pass through this one 3/4" tee on it's way back to the boiler. So with both zones running they were only getting 40,000 btus to divide between them. 3/4" pipe = 4 gpm = 40,000 btus @ 20 degree Delta T